1. Field of the Invention
The present invention relates to a magnetic sensor for detecting a change in magnetic field by detecting a corresponding change in voltage via a coil thereby detecting the position or the speed of revolution of an object.
2. Description of the Related Art
FIG. 13 is a cross-sectional view illustrating a conventional magnetic sensor in operation. FIG. 14 is a perspective view illustrating the general structure of the conventional magnetic sensor before being placed into a housing.
FIG. 15 is a cross-sectional view taken along line XV--XV of FIG. 14, and FIG. 16 is a cross-sectional view taken along line XVI--XVI of FIG. 15. The magnetic sensor 1 includes: a magnetic core 2; a bobbin 3 surrounding the magnetic core 2; a coil 4 consisting of a wire wound around the bobbin 3; a cylindrical supporting element 5 integrally connected to the bobbin 3; a magnet 6 in the form of, for example, a cylinder and a spacer 7 both placed inside the supporting element 5; a first electric terminal 8a and a second electric terminal 8b both protruding from the supporting element 5 in a radial direction; a tape 10 for pressing an input lead wire 9a and an output lead wire 9b of the coil 4 against the supporting element thereby fixing them to the supporting element 5; and a housing 11 in which the assembly 20 consisting of the above components 2, 3, 4, 5, 6, 7, 8a, 8b, and 10 are placed.
The bobbin 3 includes a winding core 15 around which the wire is wound, and flanges 12 disposed at either end of the winding core 15. One of the flanges 12 has an input lead slot 13 and an output lead slot 14 formed at locations opposite to the first electric terminal 8a and the second electric terminal 8b, respectively, wherein the input lead slot 13 and the output lead slot 14 extend in radial directions.
The process of producing the above magnetic sensor 1 is described below. First, the input lead wire 9a is put in through the input lead slot 13, and the wire is wound around the winding core 15 so as to form the coil 4 consisting of a plurality of wire layers. The output lead wire 9b extending from the outer periphery of the coil 4 is put out through the output lead slot 14 and is further placed on the supporting element 5 along its axis. The end of the input lead wire 9a is temporarily fixed to the first electric terminal 8a and the end of the output lead wire 9b is temporarily fixed to the second electric terminal 8b. The band-shaped fabric tape 10 is wound around the periphery of the supporting element 5 so that the input lead wire 9a and the output lead wire 9b are pressed against the supporting element 5 thereby fixing them to the supporting element 5. Then the end of the input lead wire 9a is firmly connected to the first electric terminal 8a by soldering. Similarly, the end of the output lead wire 9b is firmly connected to the second electric terminal 8b by soldering. The magnetic core 2 is placed into the bobbin 3, the magnet 6 and the spacer 7 are placed into the supporting element 5, and they are fixed together so that the complete assembly 20 is obtained.
The assembly 20 is then put in a mold, and a molten resin is injected into the mold so as to form the housing 11 surrounding the assembly 20 thereby obtaining the complete magnetic sensor 1.
In the magnetic sensor 1 described above, a magnetic path is formed such that a magnetic flux emerging from the magnet 6 passes through the magnetic core 2. When a signal detecting plate 16 having a projection 16a, made up of a magnetic material, is rotated, the distance between the magnetic sensor 1 and the signal detecting plate 16 varies with the rotation of the signal detecting plate 16. As a result, a change occurs in the magnetic flux emerging from the magnet 6. The change in the magnetic flux is detected as a change in voltage by the coil 4, and the signal detected by the coil 4 is transmitted to a computer unit(not shown) so that the speed of revolution of the signal detecting plate 16 is determined.
The conventional magnetic sensor 1 described above has the following problems. In order to achieve a high enough voltage for the detection signal of the magnetic sensor 1, the wire has to be wrapped around the winding core 15 many times. As a result, as shown in FIG. 15, the radius X.sub.1 of the coil 4 generally becomes greater than the radius X.sub.2 of the supporting element 5 and thus a difference in level is created between the outer periphery of the coil 4 and the outer periphery of the supporting element 5. This difference in level causes the output lead wire 9b, which extends though the output lead slot 14 and further extends along the axis of the supporting element 5, to have an oblique portion 9b.sub.1 extending away from the supporting element 5. Since this oblique portion 9b.sub.1 is separate from the outer periphery of the supporting element 5, the oblique part 9b.sub.1 by itself should withstand the injection pressure of the molten resin when the molten resin is injected into the mold to form the housing 11. As a result, great tensile stress is imparted to the output lead wire 9b, which can damage the coil 4.
It is desirable that the thickness of the flanges 12 be small enough so that the total length of the magnetic sensor 1 becomes small and the tape 10 can have a large pressing area. However, the thickness of the flanges 12 should be greater than about 1 mm so that the flanges 12 can withstand the injection pressure of the molten resin. One of the flanges 12 has the output lead slot 14 having a corner perpendicular to the winding direction of the winding wire of the coil 4. When the output lead wire 9b comes in contact with the right-angle corner, the output lead wire 9b can be damaged.
In view of the above, it is a general object of the present invention to solve the above problems.
More specifically, it is an object of the present invention to provide a magnetic sensor in which a coil does not become damaged due to the injection pressure of a molten resin.
It is an another object of the present invention to provide a magnetic sensor in which an output lead wire can be drawn out through an output lead slot without becoming damaged due to contact with a corner of the output lead slot.